DNA fragments containing gene having function relating to autonomous proliferation of plasmid

ABSTRACT

Provided are a DNA fragment containing a gene relating the autonomous replication of a plasmid, and a DNA fragment containing a gene relating to the autonomous replication of a plasmid wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene. Also provided are a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus  Rhodococcus , a DNA fragment wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene, and a plasmid carrying a DNA fragment wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site of the gene.

FIELD OF THE INVENTION

The present invention relates to a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid. The present invention further relates to a DNA fragment, which contains a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus and wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene. The present invention also relates to a multiple-copy-number plasmid vector carrying the DNA fragment.

BACKGROUND ART

Microorganisms belonging to the genus Rhodococcus are known as microbial catalysts to hydrate nitrites so as to produce corresponding amides or acids. Moreover, the microorganisms also exert a great variety of properties in producing enzymes involved in degradation of PCB (polychlorinated biphenyl) or the like or desulfurization of crude oil, as well as producing biosurfactants which are used, for example, for effluent treatment, so that they are known to be industrially very useful microorganisms.

In addition, microorganisms belonging to the species of Rhodococcus rhodochrous are known to have extremely highly efficient nitrile hydration activity, so that they are industrially utilized as catalysts for producing acrylamide through biotechnology.

The term “ATCC” where it appears in the specification below refers to the American Type Culture Collection® or ATCC®.

Under such circumstances, the development of host vector systems of the genus Rhodococcus has been expected, and some such systems have been developed. Examples of industrially useful plasmid vectors of bacteria of the genus Rhodococcus include plasmid pRC001 carried by Rhodococcus rhodochrous ATCC4276, pRC002 carried by Rhodococcus rhodochrous ATCC14349, pRC003 carried by Rhodococcus rhodochrous ATCC14348, plasmid pRC004 (see Japanese Patent No. 2983602) carried by Rhodococcus rhodochrous IFO3338, and composite plasmid vectors pK1, pK2, pK3 and pK4 (see JP Patent Publication (Kokai) No. 5-64589 A (1993)) comprising a plasmid DNA region capable of duplicating and replicating within the cells of Escherichia coli and a DNA region containing a drug resistance gene and pRC020 (see JP Patent Publication (Kokai) No. 9-28379 A (1997)) carried by Rhodococcus erythropolis IFO12320. When a useful gene is introduced into a plasmid vector and then expressed by bacteria of the genus Rhodococcus as a host, it is known that the expression level of this useful gene is almost proportional to the number of copies of the plasmid vector. However, the number of copies of these plasmids within the cells of the bacteria of the genus Rhodococcus is around 2 to 6, which is not always sufficient to cause high expression of a foreign gene.

SUMMARY OF THE INVENTION

Introducing a multiple-copy-number plasmid vector having a useful gene inserted therein into cells is expected to result in a high gene amplification effect.

An object of the present invention is to provide a DNA fragment which is derived from a plasmid capable of autonomously replicating within the bacteria of the genus Rhodococcus and containing a gene relating to the autonomous replication of a plasmid. Specifically, it is an object of the present invention to provide a DNA fragment derived from a plasmid capable of autonomously replicating within the bacteria of the genus Rhodococcus and containing a gene involved in the autonomous replication of a plasmid, wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene.

We have completed the present invention by revealing a DNA region containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus, and obtaining a DNA fragment wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene. That is, the present invention relates to a DNA fragment derived from a plasmid selected from plasmids pRC001, pRC002, pRC003 and pRC004, and containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus. The present invention further relates to a DNA fragment derived from a plasmid capable of autonomously replicating within the bacteria of the genus Rhodococcus and containing a gene relating to the autonomous replication of a plasmid, wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene. The present invention is as follows.

(1) A DNA fragment, which is derived from a plasmid selected from plasmids pRC001, pRC002, pRC003 and pRC004, and contains a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus.

(2) The DNA fragment of (1), which has a size of 1.6 kb, and has cleavage points for restriction enzymes Spl I and Sac I at its ends.

(3) The DNA fragment of (1), which has a size of 1.7 kb, and has cleavage points for restriction enzymes Sma I and Sac I at its ends.

(4) The DNA fragment of (1), which has a size of 1.9 kb, and has cleavage points for a restriction enzyme Sma I at both of its ends.

(5) The DNA fragment of (1), which has a size of 2.3 kb, and has cleavage points for a restriction enzyme Sac I at both of its ends.

(6) The DNA fragment of (1), which is contained in pRC004, contains a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus and consists of the nucleotide sequence of SEQ ID NO: 1.

(7) The DNA fragment of (1), which is contained in pRC004, contains a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus and consists of the nucleotide sequence of SEQ ID NO: 3.

(8) The DNA fragment of (1), which is contained in pRC004, contains a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus, and consists of the nucleotide sequence of SEQ ID NO: 7.

(9) The DNA fragment of any one of (1) to (8), wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site.

(10) The DNA fragment of (9), wherein a mutation point capable of increasing the number of copies of a plasmid is present at one site, and which comprises the nucleotide sequence of SEQ ID NO: 2.

(11) The DNA fragment of (9), wherein a mutation point capable of increasing the number of copies of a plasmid is present at one site, and which comprises the nucleotide sequence of SEQ ID NO: 4.

(12) The DNA fragment of (9), wherein a mutation point capable of increasing the number of copies of a plasmid is present at one site, and which comprises the nucleotide sequence of SEQ ID NO: 9.

(13) A plasmid, which carries the DNA fragment of any one of (9) to (12), is capable of autonomously replicating within the bacteria of the genus Rhodococcus, and can be present in a number of copies.

(14) A plasmid pLK006, which carries the DNA fragment of claim 11, is capable of autonomously replicating within the bacteria of the genus Rhodococcus, and can be present in a number of copies.

The DNA fragment of the present invention (hereinafter referred to as a multiple-copy plasmid DNA fragment), which contains a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus and wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene means a DNA fragment which contains a gene having a function relating to the autonomous replication of a plasmid and has an action that can increase the number of copies of a plasmid vector that is produced using a DNA fragment having a mutation point, compared with the number of copies of a plasmid vector that is produced using a DNA fragment having no mutation point. In this specification, the term “number of copies” means the number of plasmid molecules per cell (the number of existing plasmids). The term “a number of copies” means the number of copies which is greater than the number of copies of a plasmid vector that is produced using a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid and having no mutation point.

The multiple-copy plasmid DNA fragment of the present invention can be obtained from a plasmid that autonomously replicates within the bacteria of the genus Rhodococcus or a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid, both of which are appropriately treated to cause mutations.

Normally, a plasmid that autonomously replicates within the bacteria of the genus Rhodococcus or a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid is inserted by a known method into a vector plasmid having a drug resistance gene that can be a marker. Examples include pHSG298 (TAKARA SHUZO) having a kanamycin resistance gene, pHSG398 (TAKARA SHUZO) having a chloramphenicol resistance gene, pBR322 (TAKARA SHUZO) having a tetracycline resistance gene, and pUC18 (TAKARA SHUZO) having an ampicillin resistance gene. The vector plasmid was then transformed into a host bacterium of the genus Rhodococcus by electroporation method. Plasmid DNA is extracted from the resulting transformant by a known method such as an alkaline SDS method, and then a target DNA fragment can be confirmed by, for example, analyzing fragments treated with restriction enzymes.

Appropriate treatment to cause mutations means to irradiate bacteria carrying the plasmids prepared as described above with ultraviolet rays, X-rays, γ-rays or the like, or to treat the same with a mutation-causing agent such as N-methyl-N′-nitrosoguanidine. Furthermore, spontaneous mutations that occur when an organism proliferates can also be utilized.

Alternatively, a plasmid that autonomously replicates within the bacteria of the genus Rhodococcus or a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid is directly, or after treatment to make it a single-stranded DNA with hydrazine, formic acid or nitrous acid. Alternatively, it is also effective to select an appropriate primer, and then directly introduce a mutation into a DNA fragment by a known method including error-prone PCR or the like wherein PCR is performed in the presence of a nucleotide analog or a manganese ion. According to this method, a mutation can be intensively introduced at a certain position of a DNA sequence. Thus, for example, a mutation can be specifically introduced into the DNA fragment, as disclosed in the present invention, containing a gene having a function relating to the autonomous replication of a plasmid. Furthermore, a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid can also be synthesized.

A recombinant carrying a plasmid vector having a drug marker gene wherein a mutation has been introduced as described above is cultured under the selection using a drug marker at various concentrations. Thus, strains that are resistant to the drug at higher concentrations are selected, so that strains with the thus increased number of copies of the plasmid can be obtained. It can be considered that the elevated drug concentrations, to which such strains are resistant, result from a gene amplification effect brought by an increased number of copies of the plasmid. Multiple-copy plasmid DNA fragments can be obtained by collecting plasmids from such strains.

In the case of a plasmid that autonomously replicates within the bacteria of the genus Rhodococcus or a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid, which have been treated to cause mutations, the above-mentioned plasmid vector having an appropriate drug resistance gene is inserted, and then transformation of a host bacterium of the genus Rhodococcus is performed by electroporation method. The host bacteria are cultured under the selection using a drug marker at various concentrations so as to select strains that are resistant to the drug at higher concentrations. Thus, strains with an increased number of copies of the plasmid can be obtained.

It is convenient to use a drug resistance gene as a marker gene. A known method using alkaline phosphatase, luciferase, lacZ genes or the like that enables simple detection can also be used.

The number of copies can be analyzed according to, for example, a method described in Journal of Bacteriology, 152, p. 722 (1982). Specifically, analysis can be performed by extracting chromosomal DNA and plasmid DNA from cells, and then finding the ratio of the number of molecules of both DNAs. More simply, the number of copies can be compared by culturing a recombinant carrying a plasmid having no mutation point and a recombinant carrying a plasmid having a mutation point under the same conditions, extracting plasmids, performing agarose electrophoresis, staining with ethidium bromide or the like, and then analyzing densitometry.

SEQ ID NO: 1 shows the result of determining the nucleotide sequence of plasmid pRC004 derived from Rhodococcus rhodochrous IFO 3338. Based on the nucleotide sequence information, 2 open reading frames (ORF) are deduced to be present in PRC004. One ORF (the putative amino acid sequence is shown in SEQ ID NO: 6) is deduced to be encoded by 921 nucleotides (the 1142nd to the 2062nd nucleotides in the nucleotide sequence of SEQ ID NO: 1) represented by SEQ ID NO: 5. This ORF is analogous to Rep A protein involved in the replication of pKA22 plasmid derived from Rhodococcus rhodochrous NCIMB13064 and pAL5000 plasmid derived from Mycobacterium fortuitum. The other ORF (the putative amino acid sequence is shown in SEQ ID NO: 8) is deduced to be encoded by 282 nucleotides (the 2052nd to the 2333rd nucleotides in SEQ ID NO: 1) represented by SEQ ID NO: 7. This ORF is analogous to the ORF deduced to be a DNA-binding plasmid replication factor of plasmid pFAJ2600 derived from Rhodococcus erythropolis N186/21. However, the above discussion is based on genetic similarity, and their functions have not been determined. Thus they are likely to be pseudogenes that do not actually function.

According to the present invention, it was revealed for the first time that an approximately 1.6-kb DNA fragment (SEQ ID NO: 3) produced by cleavage of pRC004 with Spl I and Sac I is a region containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus.

Specifically, it was revealed that an approximately 1.9-kb DNA fragment and an approximately 2.3-kb DNA fragment that are produced by cleavage of pRC004 with Sma I and Sac I, respectively, both contain a region containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus. Furthermore, when the approximately 1.9-kb DNA fragment that had been produced by cleavage with Sma I was cleaved with Sac I, and then the produced DNA fragment of approximately 1.7 kb was examined, it was revealed that this DNA fragment also contains a region containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus. It was also revealed that an approximately 1.6-kb DNA fragment produced by cleavage of the approximately 1.7-kb DNA fragment with Spl I contains a region containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus.

When Bam HI, Bgl II, Sph I and Xho I recognition sites existing in the above DNA fragment of approximately 1.6 kb having cleavage points for restriction enzymes Spl I and Sac I at its ends were respectively cleaved with each of their corresponding restriction enzymes, treated with klenow fragments and the like for blunt-ending to perform self-ligation, the autonomous replication function of the plasmid within the bacteria of the genus Rhodococcus was lost. Thus, it was revealed that a protein factor capable of actually functioning is present in this region.

Furthermore, the finding that introduction of a mutation point to at least one site in the DNA fragment of pRC004 can increase the number of copies of a plasmid has been disclosed for the first time by the present invention.

Specifically, a multi-copy variant plasmid can be obtained for the first time according to the present invention by obtaining a variant having an improved degree of resistance against kanamycin using a composite plasmid vector prepared by inserting a plasmid vector containing a kanamycin resistance gene capable of functioning as a marker into pRC004.

The discovery for the first time that a multi-copy variant plasmid can be obtained by introducing a mutation point to at least one site in a region containing a gene that has a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus is highly significant. We believe that this means that multi-copy variant plasmids can be easily obtained based on the present invention, and using a known method.

An example of such a multi-copy variant plasmid is plasmid pLK006 obtained in the present invention. pLK006 is a variant (SEQ ID NO: 4) wherein the 1336th guanine nucleotide is substituted with a thymine nucleotide within the region represented by SEQ ID NO: 3 among regions containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus. This corresponds to the 2262nd nucleotide of the nucleotide sequence of SEQ ID NO: 1 and the 211st nucleotide of the nucleotide sequence of SEQ ID NO: 7. SEQ ID NO: 2 and SEQ ID NO: 9 respectively represent SEQ ID NO: 1 and SEQ ID NO: 7, both of which have a mutation at one nucleotide. The putative amino acid sequence encoded by ORF is derived from SEQ ID NO: 8 by mutation (substitution) of the 71st glycine to (with) serine (SEQ ID NO: 10).

In addition, plasmid pLK006 was deposited with the International Patent Organism Depositary at the National Institute of Advanced Industrial Science and Technology (Central 6, 1-1-1, Higashi, Tsukuba, Ibaraki, Japan) on Jun. 22, 2001, under the accession number of FERM BP-8085.

Furthermore, the DNA fragment of the present invention also includes a DNA fragment that hybridizes under stringent conditions to the DNA and contains a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus, or a DNA fragment that hybridizes under stringent conditions to the DNA and contains a gene that has a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus and wherein a mutation point capable increasing the number of copies of a plasmid is present in at least one site. Here, stringent conditions mean conditions wherein so-called a specific hybrid is formed, but no non-specific hybrid is formed. Under such conditions, DNAs sharing high homology, such as DNAs having homology of at least 60% or more, preferably 80% or more, and further preferably 95% or more, hybridize to each other. However, DNAs having homology lower than the above homologies do not hybridize to each other. Alternatively, another example of the stringent conditions wherein DNAs hybridize to each other is the washing condition for normal Southern hybridization consisting of 60° C., 1×SSC and 0.1% SSD, or preferably consisting of a salt concentration corresponding to 0.1×SSC and 0.1% SDS. Such DNA can be obtained by altering the nucleotide sequence of the DNA fragment of the present invention by, for example, a site-directed mutagenesis method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the restriction enzyme fragment map of plasmids pRC001, pRC002, pRC003 and pRC004.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described more specifically by the following examples. These examples are not intended to limit the technical scope of the present invention.

EXAMPLE 1

Preparation of a DNA fragment of pRC004 containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus.

Plasmid pRC004 (1 μg) extracted from Rhodococcus rhodochrous IFO 3338 was allowed to react at 37° C. for 1 hour with 5 units of restriction enzyme Sma I or Sac I that had been added thereto, so as to cleave the plasmid DNA. The solution of the plasmid cleaved with the restriction enzyme was subjected to 0.7% agarose gel electrophoresis. From the product cleaved with Sma I, approximately 650 bp or approximately 1.9-kb DNA fraction was excised. From the product cleaved with Sac I, approximately 300 bp or approximately 2.3-kb DNA fragment was excised.

In the meantime, 0.5 μg of plasmid vector pHSG299 (TAKARA SHUZO) having a kanamycin resistance gene was allowed to react at 37° C. for 1 hour with 5 units of restriction enzyme Sma I or Sac I that had been added thereto, so as to cleave the plasmid DNA. A one-tenth volume of 1 M-Tris-HCl (pH9.0) was added to the reaction solution, and then allowed to react with alkaline phosphatase (1 unit) at 65° C. for 1 hour. The solution of the plasmid vector cleaved with the restriction enzyme was subjected to 0.7% agarose gel electrophoresis, and 2.7-kb DNA fragments were excised.

Upon the excision of the DNA fragments, lambda phage DNA digested with Hind III was used as a size marker, so that the size of DNA was calculated. DNA was extracted from the agarose gel using a Gene clean kit (FUNAKOSHI), and then dissolved in TE buffer (10 mM Tris-HCl, 1 mM EDTA, (pH8.0)).

The solutions containing each DNA fragment were mixed in an equivalent volume. Then each component was added to the solution to achieve T4 DNA ligase (1 unit), 1 mM ATP, 10 mM dithiothreitol and 10 mM MgCl₂, followed by reaction at 4° C. overnight.

The above reaction solution was added to the competent cells (TAKARA SHUZO) of the Escherichia coli JM105 strain. The resultant was allowed to stand at 0° C. for 1 hour, and then subjected to heat treatment at 42° C. for 2 minutes. 2×YT medium (0.5% NaCl, 1% yeast extract and 1.6% trypton) was added to the solution, and then shaken at 37° C. for 1 hour. The solution was spread on 2×YT agar medium containing 25 μg/ml kanamycin, 1 mM IPTG (isopropyl-β-galactopyranoside) and 0.02% X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside) was coated with the solution, and then the agar medium was allowed to stand at 37° C. overnight. White colonies were selected from the colonies that had appeared, and then shake-cultured in 2×YT medium (3 ml) containing 50 μg/ml kanamycin at 37° C. for 8 hours.

Cells were collected by centrifugation at 15,000 rpm for 5 minutes, and then suspended in 0.35 ml of STET solution (8% sucrose, 0.5% TritonX-100, 50 mM EDTA and 10 mM Tris-HCl (pH8.0)). 25 μl of lysozyme solution (10 mg/ml) was added to the suspension. After agitation with Vortex for 3 seconds, the product was immersed in boiling hot water for 50 seconds. Centrifugation was performed at 15,000 rpm for 15 minutes to remove the precipitate, so that the supernatant was obtained. 0.5 ml of TE saturated phenol:chloroform (1:1) solution was added to the supernatant. After agitation, centrifugation was performed at 15,000 rpm for 5 minutes, so that the upper layer was obtained. 0.5 ml of diethyl ether was added and mixed, and then centrifugation was performed to remove the upper layer. 0.5 ml of isopropanol and 50 μl of 2.5 M sodium acetate solution (pH 4.5) were added, and then the solution was allowed to stand at −80° C. for 30 minutes. Then, centrifugation was performed at 15,000 rpm for 10 minutes, so that the precipitate was obtained. The precipitate was washed with 70% ethanol, dried under reduced pressure, and then dissolved in 0.1 ml of TE buffer.

Using the above plasmid solution prepared as described above, cleavage was performed with restriction enzyme Sma I or Sac I, thereby confirming that the target DNA fragments had been inserted. Using plasmids each having a different DNA fragment inserted therein, and Rhodococcus rhodochrous ATCC12674 as a host, transformation was performed by electroporation method.

Transformation according to electroporation method was performed by the following methods. Cells at the logarithmic growth phase of the Rhodococcus rhodochrous ATCC12674 strain were collected with a centrifugal separator, washed 3 times with ice-cooled sterilized water, and then suspended in sterilized water. 1 μl of the above plasmid and 10 μl of the cell suspension were mixed, and then ice-cooled. The suspension of DNA and the cell was put into a cuvette, and then subjected to electric pulse treatment (at 2.0 kV and 200 OHMS) using a gene transfer system (Gene Pulser (BIO RAD)). The solution treated with electric pulses was allowed to stand for 10 minutes while cooling with ice. Then heat shock was performed at 37° C. for 10 minutes, and then 500 μl of MYK medium (0.5% polypeptone, 0.3% bactoyeast extract, 0.3% bacto malt extract, 0.2% K₂HPO₄ and 0.2% KH₂PO₄) was added. After the solution had been allowed to stand at 30° C. for 5 hours, the solution was spread on MYK agar medium containing 50 mg/L kanamycin and then the agar medium was allowed to stand at 30° C. for 3 days.

Table 1 shows plasmids used herein and whether or not transformants could be obtained. When a plasmid prepared by inserting an approximately 1.9-kb Sma I-cleaved fragment and an approximately 2.3-kb Sac I-cleaved fragment of pRC004 into a plasmid vector pHSG299 was used, a kanamycin-resistant recombinant was obtained. Plasmids were extracted from the obtained recombinants, and then analyzed by agarose electrophoresis. Thus, plasmids each having the same molecular weights as those of the plasmids used for transformation were obtained.

TABLE 1 Whether or not kanamycin-resistant Plasmid used transformant was obtained Sma I  650 bp + pHSG299 not obtained Sma I  1.9 kb + pHSG299 obtained Sac I  300 bp + pHSG299 not obtained Sac I  2.3 kb + pHSG299 obtained

EXAMPLE 2

In a manner similar to that of Example 1, plasmid pRC004 (1 μg) extracted from Rhodococcus rhodochrous IFO 3338 was allowed to react at 37° C. for 1 hour with 5 units of restriction enzymes Sma I and Sac I that had been added thereto, so as to cleave the plasmid DNA. The solution of the plasmid cleaved with the restriction enzymes was subjected to 0.7% agarose gel electrophoresis, so that an approximately 1.7-kb DNA fragment was excised.

In the meantime, 0.5 μg of plasmid vector pHSG299 (TAKARA SHUZO) having a kanamycin resistance gene was allowed to react at 37° C. for 1 hour with 5 units of restriction enzymes Sma I and Sac I that had been added thereto, so as to cleave the plasmid DNA. A one-tenth volume of 1M-Tris-HCl (pH 9.0) was added to the reaction solution, and then the resultant was allowed to react with alkaline phosphatase (1 unit) at 65° C. for 1 hour. The solution of the plasmid vector cleaved with the restriction enzymes was subjected to 0.7% agarose gel electrophoresis, so that a 2.7-kb DNA fragment was excised.

The solutions each containing different DNA fragments were mixed in an equivalent volume, and then each component was added to the solution to achieve T4 DNA ligase (1 unit), 1 mM ATP, 10 mM dithiothreitol and 10 mM MgCl₂. The solution was allowed to react at 4° C. overnight.

Escherichia coli JM105 was transformed with the above reaction solution, so that a plasmid having the above DNA fragment of approximately 1.7 kb inserted therein was obtained. Rhodococcus rhodochrous ATCC12674 was transformed by electroporation method using the obtained plasmid, so that a kanamycin-resistant transformant could be obtained.

EXAMPLE 3

The plasmid obtained in Example 2 was allowed to react at 37° C. for 1 hour with 5 units of restriction enzymes Spl I and Sma I that had been added thereto, so as to cleave the plasmid DNA. The cleaved plasmid was collected by ethanol precipitation and blunt-ended by the addition of 5 units of klenow fragments. Each component was added to the product to achieve T4 DNA ligase (1 unit), 1 mM ATP, 10 mM dithiothreitol and 10 mM MgCl₂, followed by reaction at 4° C. overnight.

Escherichia coli JM105 was transformed with the above reaction solution, so that a plasmid having the above DNA fragment of approximately 1.6 kb inserted therein was obtained. Rhodococcus rhodochrous ATCC12674 was transformed by electroporation method using the obtained plasmid, so that a kanamycin-resistant transformant could be obtained.

EXAMPLE 4 Obtainment of Multi-Copy Plasmid DNA Fragment

Rhodococcus sp N775 (deposited with the International Patent Organism Depositary at the National Institute of Advanced Industrial Science and Technology (Central 6, 1-1-1, Higashi, Tsukuba, Ibaraki, Japan) on Jan. 10, 1986, under the accession number of FERM BP-961) was transformed by the electric pulse method using a composite plasmid vector pK4 consisting of plasmid pRC004 and plasmid pHSG299. The obtained transformant was cultured in 10 ml of MYK medium at 30° C. for 1 day, and then irradiated with ultraviolet rays within a clean bench so as to perform treatment to cause mutations. The treated culture solution was spread on MYK agar medium containing 50 to 400 μg/ml kanamycin, and then the agar medium was allowed to stand at 30° C. for 3 days.

Colonies that had grown were cultured, so that plasmids were extracted. Rhodococcus sp N775 was transformed again using the extracted plasmid, and then it was checked whether or not the transformant was resistant to kanamycin at a higher concentration. Several strains of the recombinants having clearly improved resistance to kanamycin, that is, those resistant to higher kanamycin concentrations, were obtained. Chromosomes and plasmid DNAs were prepared from the pLK006 strain, one of the obtained recombinants, and the recombinant transformed with composite vector pK4 having no mutation points. When the number of copies of the plasmids was then compared, the number of copies in pLK006 obtained by treatment to cause mutations was found to be elevated to a level approximately 5 times greater than that of pK4.

Of the above microorganisms, gene recombinants that we had prepared were deposited under the above number with the International Patent Organism Depositary at the National Institute of Advanced Industrial Science and Technology.

EXAMPLE 5 Determination of Mutation Point

The nucleotide sequence of the multi-copy variant plasmid pLK006 obtained in Example 3 was determined using a fluorescence sequencer ALFII (Pharmacia). As a result, the nucleotide sequence represented by SEQ ID NO: 4 was obtained.

In the region represented by SEQ ID NO: 2, the 1336th guanine nucleotide was mutated to (substituted with) a thymine nucleotide.

INDUSTRIAL APPLICABILITY

Introduction of a useful gene into the multi-copy-number plasmid vector, which is provided by the present invention, using the bacterium of the genus Rhodococcus as a host makes it possible to improve the expression level of the useful gene. The thus obtained gene recombinant having the improved expression level of a useful gene is industrially very useful.

All publications, patents and patent applications cited herein are incorporated herein by reference in their entirety. It is therefore readily to be understood by a person skilled in the art that numerous modifications and variations of the present invention are possible within the scope of the invention without departing from the technical idea and the scope of the invention as described in the appended claims. The present invention is intended to encompass such modifications and variations. 

1. An isolated polynucleotide which is at least 95% homologous to SEQ ID NO: 7 or which hybridizes to the full complement of SEQ ID NO: 7 under stringent conditions, wherein stringent conditions comprise washing in 0.1×SSC and 0.1% SDS at 60° C.; wherein said isolated polynucleotide contains a point mutation of the polynucleotide sequence of SEQ ID NO: 7 and the point mutation changes the 71^(st) Gly of the amino acid sequence of SEQ ID NO: 8 to Ser, and wherein the isolated polynucleotide increases plasmid copy number when incorporated into plasmid in Rhodococcus when compared to the plasmid copy number in Rhodococcus containing a plasmid having the unaltered polynucleotide sequence of SEQ ID NO:
 7. 2. The isolated polynucleotide of claim 1, which comprises a point mutation at nucleotide 211 of SEQ ID NO: 7 so that nucleotide 211 is thymine.
 3. A vector comprising the isolated polynucleotide of claim 1 or
 2. 4. An isolated host cell comprising the vector of claim
 3. 5. The isolated host cell of claim 4 which is Rhodococcus.
 6. A method for increasing plasmid copy number in Rhodococcus comprising transforming a Rhodococcus cell with the vector of claim
 3. 7. An isolated polynucleotide which comprises SEQ ID NO:
 2. 8. An isolated polynucleotide which comprises SEQ ID NO:
 9. 9. A vector comprising the isolated polynucleotide of claim 7 or
 8. 10. An isolated host cell comprising the vector of claim
 9. 11. The isolated host cell of claim 10 which is Rhodococcus.
 12. A method for increasing plasmid copy number in Rhodococcus comprising transforming a Rhodococcus cell with the vector of claim
 9. 13. Plasmid pLK006 identified by accession number FERM BP-8085. 